Research scientists are using middens - material encased in ancient rodent urine - to chart thousands of years of climate evolution

Tate Williams

Local researchers have compiled a detailed 22,000-year history of climate change in one of the world's driest deserts by dating fossilized rodent middens.

Scientists from the University of Arizona and the U.S. Geological Survey on Tumamoc Hill examined the middens - clods of vegetation preserved in crystallized rodent urine - along with preserved deposits from dried springs. They used them to create a detailed record of climate change in the hyperarid Atacama Desert in Chile.

The study, published this month in the journal Science, found that the desert's climate fluctuated, including a wet period about 10,000 years ago that appeared to be connected to climate changes in distant areas.

The study will help researchers understand how climate changes are connected to each other, said Julio Betancourt, the USGS researcher who led the project.

Specifically, the midden findings suggest that changes in tropical systems may affect the Earth's climate in relatively short periods of time.

"We see a lot of symptoms of major changes on very short time scales," said Jay Quade, a UA geoscientist who joined Betancourt in the desert research. Claudio Latorre of the University of Chile, UA graduate student Jason Rech and Kate Rylander, lab manager at Tumamoc Hill, also took part in the research.

With this study and ongoing data collection in the Atacama, the team will try to connect climate fluctuations in the tropics to major changes in other areas, he said.

Climatologists strive to determine what causes shifts in global climate, such as ice ages, and whether they will occur in the future.

Researchers have recently turned toward the tropical Pacific to better understand how these rapid, large-scale changes occur, Betancourt said. Events like El Nino suggest that temperatures in the tropics can affect the rest of the world very quickly, he said.

"The tropics are believed to be an important source of global climate variability at various time scales, yet their role is poorly understood," the researchers wrote in Science.

The problem with studying the tropics is that frequent rainfall and dense plant life make it tough to gather data, Betancourt said. He set out in 1994 for South America with the midden technique.

"In the Atacama, we hit pay dirt," Betancourt said.

The Atacama is a barren landscape called an absolute desert - the driest desert in the world, he said. There are parts that show no physical evidence of water or vegetation and that haven't received rain for several years.

"It's a pretty cool place - incredible landscapes, almost Martian," Betancourt said. The unfamiliarity of the desert is so jarring that he sometimes became dizzy while adjusting to the region, he said.

As strange as it may seem to study tropical systems in a barren desert, the Atacama borders the Andes Mountains, making a sharp border of radically different climates, Quade explained. To the east of the mountains lies the Amazon Basin and a large amount of tropical rainfall.

The foothills of the Andes, which are loaded with dry springs and rodent middens, reflect the changes that occur in the mountains nearby, he said.

If there is a particularly wet period in the tropical area, it will appear in the springs nearby, and rodent middens will be more frequent and filled with vegetation, Quade said.

By radiocarbon-dating middens and deposits in dried springs, the team found evidence of a wet period that ended about 10,500 years ago, he said. This was around the end of the last ice age.

The data are very readable. The Atacama is so dry that the middens and water deposits are remarkably preserved, Quade said. Also, a tropical region has fluctuations in rainfall that make it difficult to put any tracking in perspective.

The techniques were so effective that the team could see what seasons got more rain, Betancourt said.

The record the team has compiled is a remarkable feat, said Tom Van Devender, senior research scientist at the Arizona-Sonora Desert Museum. He has worked with rodent middens in the Southwest since 1969.

"It's not very well understood what happened on either side of the Andes," he said. "I don't know any equivalent record in the world."

Using middens, Van Devender worked alongside Betancourt and ecologist Paul Martin to chart 40,000 years of climate in the Southwest.

Middens are abundant in caves and can be left by several small animal species, providing a versatile way to reconstruct the history of a region.

"The best way to do it is just ask the rats," Quade said.

Betancourt and his colleagues are expanding the use of the techniques to the entire Atacama - the original study covered only a section of the desert - and other areas of the world.

The team's Atacama Desert research was funded by the National Geographic Society.